Permanent magnet alternator



April 27 1965 R. J. sHAr-RANEK ETAL 3,181,018

PERMANENT MAGNET ALTERNATOR Filed Jan. 23, 1961 United States Patent O 3,181,918 PERMANENT MAGNET ALTERNATGR Robert J. Shafranek, Kent, lanis Tiltins, East Cleveland,

and `loseph C. Lash, Cleveland, Ohio, assignors to Thompson Ramo Wooldridge Inc., Cleveland, Ghio, a

corporation of Ohio Filed Jan. 23, 1961, Ser. No. 84,102 12 Claims. (Cl. S16-156) This invention relates to an alternator assembly which is particularly designed for use in various types of aircraft such as airplanes, rockets, missiles and the like, although various features of the invention may have general application.

In any alternator system, and particularly in aircraft systems, it is desirable to have the highest possible ratio of power output to weight or size and it is desirable to have stable, reliable and efficient operation. It is therefore a general object of this invention to provide an alternator having these desirablefeatures.

y It is a further highly important object of the present invention to provide a permanent magnet alternator assembly having adequate overload capability to prevent demagnetization resulting from transient short circuit cutrent while yet meeting stringent requirements as to rotor size and inertia and rotor weight.

Another important object of the present invention resides in the provision of a permanent magnet alternator having a large leakage permeance for minimizing demagnetizatio-n due to large stator magnetomotive forces.

A further object of the invention resides in the provision of a rotor for a permanent magnet alternator accommodating a magnet material having lower tensile strength.

till another object of the invention is to provide a rotor construction for a permanent magnet alternator which affords a higher magnetic flux density output from the rotor magnet.

Other objects, features and advantages of the present invention will become apparent from the following detailed descriptiontaken in connection with the accompanying drawings, in which:

FIGURE l is a longitudinal sectional View of an alternator assembly in accordance with the present invention;

FIGURE 2 is a cross sectional view taken generally along the line II-II of FIGURE l.

FIGURE 3 is a cross sectional view taken along the line III-III of FIGURE 1;

FIGURE 4 is a somewhat diagrammatic longitudinal sectional view showing the construction of the stator;

FIGURE 5 is au elevational view showing one stator lamination; y

FIGURE 6 is a fragmentary cross sectional View showing details of the stator assembly;

FIGURE 7 is a diagrammatic perspective View of one stator winding group; yand FIGURE 8 is a graph illustrating the operating characteristics of the permanent magnet of the alternator assembly` f The illustrated alternator assembly may advantageously comprise a component of a unitary power generating and control system driven by means of a turbine wr ich may be carried directly on the shaft 1li. A system of this type is illustrated in Robert L. Anderson application Serial No. 656,646, filed May 2, 1957 and entitled Alternator Assembly and the disclosure of said Anderson application is incorporated herein by reference to provide an illustrative overall environment in which the alternator of the of the present invention and may also drive one or more auxiliary alter. ators such as a parasitic load alternator P TCC (not shown) the rotor of which may be xed to shaft 19. The parasitic alternator may function to control the speed of operation by providing an increasing load on the turbine or other source of motive power in response to increased speed of rotation of the shaft 16. The source of motive power must, of course, be such that its speed of operation is decreased when the load on shaft 1'@ is increased. If the speed of shaft 10 decreases, the parasitic alternator 14 is so controlled as to supply a decreased amount of power to its load thereby tending to restore the desired speed of operation. Suitable bearing means (not shown) may be provided for the shaft 1t) to the left of the parasitic alternator.

The primary alternator 12 ofthe present invention may be a three phase, 400 cycle per second grounded type having a suitable passive network connected in its output circuit to provide optimum voltage regulation characteristics as described in said Anderson application.

The shaft 1d is preferably operated at a high speed such as 24,060 revolutions per minute to provide a maximum ratio of power output to size and weight and the rotor 11 may have two poles to develop the 40G cycle per second electrical output. Where a turbine is utilized to drive the shaft lil, high speeds are advantageous since the turbine provides a higher efficiency and a higher power output at a high rate of speed. A housing 36 may enclose the various parts including the turbine (not shown) and the alternator 12. The cylindrical portion 36a of the housing has an open end of diametric extent to receive the stator laminations 32 whose outside diameter is substantially equal to the minimum inside diameter of the cylindrical portion Stia of the housing from the open end thereof. After assembly of the stator 32, the housing open end is closed by means of .a suitable end plate (not. shown).

The housing Tit) further includes an internal boss or wall 3tlb which is axially bored to provide a shoulder 34 to receive and abut the outer ring 35 of a ball bearing assembly indicated generally at 36. The ball bearing assembly 36 is retained in place by a clamping ring 40 secured to the boss S-tb by screws such as indicated at 42 threadedly engaging the clamping ring 40 so as to draw the ring toward the Wall Sb in clamping the outer ring 35 of the ball bearing assembly in place. The inner race 44 of the bearing assembly snugly ts into a reduced portion 46a of a shaft 46 secured to the opposite end of the rotor assembly 11 and abuts a shoulder 46b of the shaft. Balls 48 between the races 35 and 44 provide the bearing support for the shaft 46. It will be noted that the stator assembly seats against an internal shoulder 30e of the housing 30 and is secured in place by means such as a set screw indicated at 50 in FIGURE l.

The alternator 12 is of a permanent magnet type and the stator 32 may have a series of windings 52 associated with a stack of laminations of magnetic material. By Way of example, a suitable stator assembly has been illustrated in FIGURES 4, 5 and 6. Each individual lamination S4 may have a series of 24 slots 54a spaced about the inner periphery thereof to define a series of 24 pole portions 54h having small gaps 66 therebetween. As indicated in FIGURE 4, the pole portions 5417 of the successive laminations are circumferentially offset so that the gaps 66 dene slots such as indicated at 62 in the diagrammatic illustration of FIGURE 4. The slots 62 may be skewed by a circumferential distance equal to twice the separation between successive slots over the axial extent thereof. In one embodiment in accordance with the present invention the laminations were approximately .007 inch thick and about 393 laminations were stacked to provide the stator assembly. The outside diameter of each lamination was approximately 3.612 inches while the inside diameter was approximately 1.882 inches.

arancia The gaps 611 had an extent of approximately .060 inch and the slots 54a had a depth in the radial 'direction of approximately i197 inch and a width in the circumferential direction tapering from about .218 inch to about .166 inch. Each of the slots 62 is provided with suitable insulating strips such as indicated at 64 and 66 in FIG- URE 6 for proper insulation of the stator winding 52. A pair of insulator laminations as indicated Vat '711-73 may be provided 'at the respective ends of the magnetic laminations 54 and insulator laminations 741-73 may be approximately .031 inch thick and may have 24 slots equally spaced about the interior periphery thereof registering with the grooves 62 and having a Width of approximately .218 inch and a radial depth of approximately .174 inch, with the interior diameter lof the insulator laminations being approximately 2.000 inches. Thus, the bottoms of the grooves in the insulation laminations 711-73 are on the same diameter substantially as the bottom 54e of the slots 54a.

The stator winding may comprise three phase groups with eight coils per group. FIGURE 7 illustrates one group having a series of eight coils 811-87 with a lead 9i) connected with the beginning of coil 80 and a lead 91 extending from the last coil 87 and leads 92-98 connecting the successive coils Sti-87 in series between leads 90 and 91. By Way of example each coil may comprise two wires connected in parallel and wound six turns per coil. From FIGURE 6, it will be seen that each slot may receive 24 wire cross sections corresponding to two coil cross sections per slot. As indicated in FIGURE 4, the output from the stator may be by means of a common neutral conductor 100 connected with one lead of each coil group and three output conductors 1111, 1112 and 103 each connected to the other output lead of one of the coil groups 'to provide a three phase output from the alternator.

' Thev rotor assembly 11 in the killustrated embodiment comprises a pair of half sections 1111 and 111 of permanent magnet material having planar mating faces 110eV and 111e in contact along the longitudinal axis of Vthe rotor. By virtue of the flat planar surfaces 11M and 111e, the permanent magnetsections 111B and 111 may be of a` grain oriented permanent magnet'material such Vas Alnico V v(DG) which has a composition of 24% cobalt, 14% nickel, 8% aluminum, 3% copper by weight and the remainder iron. In the present instance, the grain structure of the magnetic material is oriented in the direction of the arrows 113 and 114l in FIGURE 2 which is the direction of magnetization of the material. tion of a grain oriented magnetic material requires contacting of the material by means of chill plates which can may be approximately 2.875 inches. As seen in FIGURE 2, the magnets are of generally rectangular' cross section atthe central portions thereof bounded by the iiat surfaces 110e, 11nd, 111C and 111d and end faces such as* indicated at 111eV and 11Min FIGURE 1. VThis is highly advantageoussince optimum utilization of permanentV magnet Imaterial is obtained when the magnet is of s ubstantially constant cross section in the directionof magnetization. The present invention not only provides more efficient utilization of permanent material but further the volumel of permanent'm'agnet material which can be used for a given size rotor is increased by utilizing a solid permanent magnet configuration Vas shown in FIGURE Zrather than van Vannular configuration as illustrated in the aforementioned Andersonapplication Serial No.

. 656,646. The increasev in volume of permanent magnet material is achieved without any appreciable increase in weight since the'rotor shaft does not extend `through the Forma- 121D and 121 having flat faces 12116: and 12M mating with Y the flat faces'11t1c, 111C and 1100?, 111d of the permanent magnet sections, and having outer curved sides 121111 121b defining with the magnet curved sides such as 111th a cyclindrical configuration. Electrically condutive disks `123 and 124 are disposed in mating engagement with the planar end faes 111e andi111f of the permanent magnet section 111 and the corresponding end faces of the magnet section 110, and thesedisks are electrically and mechanically connected to the bars 12),v and 121 by means of circular welds 126 and `12.7. By way of example, the damper bars 121), 121'and disks 123 and 124 may be of aluminum and the welds 126 and 127"may also be aluminum.

To provide the required rotor flux leakage at :short circuit conditions, parasitic air gaps are used in a retain ing ring that surrounds the permanent magnet structure. The retaining ring is comprised of a pair of half sleeves 14@ and 141 of magnetic material which are brazed together at their mating surfaces by means of a suitable nonrnagnetic brazing material such as copper to provide respective longitudinal gaps 143 and 144 in the magnetic circuit surrounding the permanent magnet sections and 111. To rigidly determine the spacing between the stabilizing sleeve portions 141i and 141, spacer platessuch as indicated at rss, 147, Maand 14a are interposed Y between the longitudinal edges of the sleeve portions ati the opposite axial ends thereof. These plates may have a width or axial extent of approximately .188 inch, a height or radiall extent of approximately .265 inch and a thickness of approximately .Q4-60 inch. The spacer plates may be of stainless steel or other non-magnetic material and thus define part of the nonarnagnetic longitudinally extending gaps between the sleeve portions 1%land 141.

The` retaining ring segments 14@ and 1211 may be of a suitable magnetic steel material and may bei welded to shaft portions 1G and 46 axially outwardly of the respective spacers 1146-1119 as indicated at 152 and 153'in FIG- URE l'. The shaft sections 1 6 and 46 are also of a suitable magnetic steel materialt'o provide portions of the stabilizing' leakage path about the rotor. Non-magnetic bands as indicated at 16@ and 161 are mounted on the flange parts 163 and 164 of the shaft sections 1@ and 46 with the inner axial edge faces of the Vbands 16t1and 161 flush with the inner end faces 163g and164a of the shafts 1 6 and 46. The welds 152 and 153 may be of. non! magnetic material and may serve to secure Vthe retaining ring parts 14%.and 141'to each other and to the flange parts 163 and 164'of`the shaft portions 1@ and46l Y It may be noted that the disks 123. and 124 may be suitably soldered to the end faces suchas111e and111f of the permanent magnet sections 1111 and11-1 and sim-v ilarly the damper bars 121) and 12,1 may besuitably-soldered to surfaces 111%,1110 and 11M, 11.161 ofthe permanent magnetsections. i

yFlGURE 8v shows the operating characteristics of the permanent magnet for short circuit' stabilization. Line OA represents the noload in-stator permeance produced by v(l) the radial air gap clearance between the rotor assembly11 and the stator laminations 32. and (2) the magnetic circuit of the stator laminations 32. Y Line OC represents the rotor leakage permeance established by l) c the effective parasitic air gap produced by thenon-magnetic braze sections 143 and 144, FGURE 2, (2) the effective parasitic air gap produced by the non-magnetic sections and 161, FIGURES 1 and v3, and (3) the magnetic sections 1441 and 141 of the retaining ring. i

When fully charged, the magnet has a Bmgx flux density as shown and Ywill stabilize at point K when the charge isv 5 removed. Application of a short to the alternator leads demagnetizes the rotor magnets to point C. At this point, the magnets operate at a flux density equal to line CJ; however, all of this flux is rotor leakage tlux. A negligible amount of stator useful ilux is required to circulate the short circuit current.

Removal of the short circuit allows the magnet to recover along a minor hysteresis loop represented by line CA. This is the operating line following short circuit stabilization. At no load, the operating point is at A. The total magnet density is given by line AD. The stator useful iiux producing output power is given by AF and the rotor leakage liux is reperesented by FD. Application or full load produces demagnetizing force due to the stator Winding ampere turns and is shown as 0H. This changes the operating point from A to E along the operating line. The stator useful ilux is now represented by BG, and the rotor leakage ux by GE. Additional application of load Will drive the operating point from B to C along the operating line. When short circuit conditions are reached, the magnet density is again given as Cl which is entirely rotor leakage liux. Removal of the short allows the magnet to recover as before, along the operating line. Thus from no load to short circuit, the magnet density varies only from AD to C3. The parasitic iiux paths in the rotor provide leakage paths at short circuit condition that prevent the magnet density from decreasing below the level of Cl.

An important advantage or" the illustrated construction resides in the tact that the magnetic material including shell parts Mtl and Ml and end franges id?, and lod serve to hold the permanent magnet assembly together, the respective parts being Welded together. Thus, there is no reliance on the permanent magnet material for strength and the rotor assembly can be used up to very high speeds, for example 48,960 revolutions per minute with an outside diameter of approximately l7/s inches.

To prevent a stator tooth from bridging across the spacers 143 or 144 of the stabilizing sleeve, the sleeve can be latted to a depth of about .O20 inch to .G30 inch from a circular coniiguration Where indicated by reference numerals Zilli and Ztll in FiGURE 2. Even Without the stabilizing sleeve including parts llill and 141, the rotor configuration of the present invention provides better voltage regulation than is obtained with an annular type permanent magnet. The specific dimensional embodiment described herein provides a two pole alternator having an output of approximately 1000 Watts at .85 power factor lagging.

It may be noted that the `magnet sections may be copper plated on all surfaces except the mating planar surfaces lliia and lilla prior to the soldering thereof to the damper Winding parts 126, 12.1, 123 and 124.

It will be understood that many modications and variations may be effected Without departing from the scope of the novel concepts of the present invention.

We claim as our invention:

l. An electric machine comprising a rotor assembly having permanent magnet means and mounted for rotation on an axis extending centrally of said permanent magnet means, sleeve means surrounding said permanent magnet means and having an interior surface conforming with exterior surface portions of said permanent magnet means, said sleeve means providing segmental cylindrical magnetic poles, and means of magnetic material at opposite ends of said sleeve means providing leakage iiux paths of predetermined permeance for short circuit stabilization of said machine.

2. An electric machine comprising a rotor assembly having permanent magnet means and mounted for rotation on an axis extending centrally of said permanent magnet means, sleeve means surrounding said permanent magnet means and having an interior surface conforming with exterior surface portions or" said permanent magnet means, said sleeve means providing segmental cylindrical magnetic poles, and means providing short circuit stabilization of said rotor assembly and comprising damper Winding means encircling said permanent magnet means Within said sleeve means and providing a substantially continuous closed loop conductive path of substantial cross section and formed substantially entirely of electrically conductive parts having a substantially greater conductivity than the conductivity of the material of said permanent magnet means, said parts being secured together oy electrically conductive Welds.

3. An electric machine comprising a rotor assembly having permanent magnet means and mounted for rotation on an axis extending centrally of said permanent magnet means, sleeve means surrounding said permanent magnet means and having an interior surface conforming with exterior surface portions of said permanent magnet means, said sleeve means providing segmental cylindrical magnetic poles and containing non-magnetic gaps, and stub shafts having flange means of magnetic material secured to the opposite ends of said sleeve means, said flange means being of magnetic material to provide leakage flux paths for short circuit stabilization of said machine.

4. An electric machine comprising a rotor assembly having permanent magnet means and mounted for rotation on an axis extending centrally of said permanent magnet means, said permanent magnet means having hat chordal exterior surface portions and curved outer surface portions connecting said flat surface portions, and means providing short circuit stabilization of said rotor assembly and comprising damper Winding means encircling said permanent magnet means and comprising damper bars extending along said flat chordal exterior surface portions and end disks and Welds enclosing ends of said permanent magnet means.

5. An electric machine comprising a rotor assembly having permanent magnet means and mounted for rotation on an axis extending centrally of said permanent magnet means, said permanent magnet means having flat chordal exterior surface portions and curved outer surface portions connecting said lat surface portions, damper Winding means encircling said permanent magnet means and comprising damper bars extending along said hat chordal exterior surface portions and being in engagement with said dat chordal exterior surface portions, and magnetic sleeve means encircling said permanent magnet means containing non-magnetic segments and having interior surface portions overlying said curved outer surface portions of said permanent magnet means and overlying exterior surfaces of said damper bars.

6. An electric machine comprising a rotor assembly having permanent magnet means and mounted for rotation on an axis extending centrally of said permanent magnet means, said permanent magnet means having liat chordal exterior surface portions and curved outer surface portions connecting said iiat surface portions, and damper winding means encircling said permanent magnet means and comprising damper bars extending alon r and engaging said ilat chordal exterior surface portions, and electrically conductive members enclosing opposite ends of the permanent magnet means and electrically and mechanically connected with the respective adjacent ends of said damper bars, said damper bars providing a substantially continuous conductive path of substantial crossection between the conductive members, and the conductive members and damper bars being connected by Welds of electrically conductive material.

7. An alternator assembly comprising permanent magnet rotor means mounted for rotation on a longitudinal axis and stator means encircling said rotor means for generating an electric current in response to rotation of said rotor means, said rotor means comprising sleeve means of generally cylindrical and segmented configuration encircling said axis, stub shafts secured to the opposite ends of said sleeve means for mounting said sleeve means for rotation on said axis, and a permanent magnet havmg curved exterior surface portions snugly tting within said 8. An alternator assembly comprising permanent mag- Y net'roto'r means mounted for rotation on a longitudinal axis and stator means encircling said rotor means for generating an electric current in response to rotation of said rotor means, said rotor means comprising sleeve means Vofy generally cylindrical configuration encircling said axis, stub shafts secured tothe opposite ends of said sleeve means for mounting said sleeve means for rotation'on said axis, and a permanent magnet Within said sleeve means, said sleeve means and saidstub shafts providing short circuit stabilizing magnet leakage paths for said assembly, and damper Winding vmeans encircling said permanent magnet sections Within said sleeve means and Within said stub shafts.

9. An electric machine'comprising a permanent magnet rotor assembly comprising a permanent magnet, stub shafts at the opposite ends of the permanent magnet rotor assembly, and sleeve means comprising segments of magnetically soft material encircling the permanent magnet of said assembly, said stub shafts having anged end portions of magnetically soft material Welded to said sleeve means to provide a rigid casing for said permanent magnet.

10. AnV alternator assembly comprising permanent magnet rotor means mounted for rotation on a longitudinal axis and stator means encircling said rotor means for generating an electric current in response to rotation of said rotor means, said rotor means comprising sleeve means of generally cylindrical configuration encircling said axis and comprising segments of magnetically soft material, Stub shafts of magnetically soft material secured to the opposite ends of said sleeve means for mounting said sleeve means for rotation on said axis, and a permanent magnet having curved exterior surface portions snugly tting Within said sleeve means.

1l. ln an alternator assembly having permanent magnet rotor means-for rotation on a longitudinal axis at a SpeedA at least of the order of 24,000 revolutions per minute and stator means encircling said rotor means for generating an electric output current in response to rotation of said rotor means, said rotor means comprising sleeve means encircling said axis and comprising a plurality of segments of magnetic material having gaps of non-magnetic material therebetween, permanent magnet means Within said sleeve means and having respective polar faces conforming with and confronting respective ones of said segments, and means providing short circuit stabilizationof said rotor means comprising damper winding means providing a continuous closed loop path of electrically conductive material encircling said permanent magnet means and comprising electrically conductive Imaterial extending axially along said permanent magnet al? means at opposite sides of the polar faces thereof and comprising electrically conductive non-magnetic material at the opposite ends of the permanent magnet means, and members of magnetically soft magnetic material at the opposite ends of the permanent magnet means and Vaxially separated from the ends of the permanent magnet means by said electrically conductive nommagnetic material and providing magnetic leakage flux paths between said segments of said sleeve means.

12. In an alternator assembly having permanent magnet rotor means for rotation on a longitudinal axis at a speed at least of the order of 24,000 revolutions per minute and having'means encircling said rotor means for generating an alternating current of a frequency at least of the order of 400 cycles per second in response to rotation of said rotor means, said rotor means consisting es-k sentially of sleeve means of cylindrical configuration and encircling said axis and comprising a plurality of segmental cylindrical sleeve parts of magnetic material having exterior and interior segmental cylindrical surfaces and non-magnetic material rigidly connecting said sleeve parts to provide said sleeve means with substantially smooth continuous exterior and interior cylindrical surfaces, permanent magnet means Within said sleeve means and having respective segmental cylindrical polar faces conforming with and confronting respective segmental cylindrical interior surfaces of said segmental sleeve parts of magnetic material, said sleeve means conlining said permanent magnet means and providing mechanical support therefor during rotation of said rotor means, and means providing short circuit stabilization of said rotor means comprising damper Windingrmeans providing a continuous closed loop path of electrically conductive non-magnetic material encircling said permanent magnet means and comprising electrically conductive bars at least substantially completely filling the spaces between the permanent magnet means and theinterior cylindrical surface of said Sleeve means and electrically conductive non-magnetic plates at the opposite axial ends of the permanent magnet means, and members of magnetically soft magnetic material at the opposite ends of the permanent magnet means but separated therefrom by said electrically conductive non-magnetic plates, and mechanically connected with said sleeve means to provide a rigid casing for said permanent magnet means and providing magnetic leakage flux paths between said segmental sleeve parts of magnetic material.

References Cited by the Examiner UNTED STATES PATENTS 2,863,077 12/58 Morrill 310-156 2,927,229 3/60 Merrill 3l0-156 3,077,026 2/ 63 Blackburn 310-156 FOREIGN PATENTS 7 49,480 f 5 56 VGreat Britain.

MILTON O. HlRSHFIELD, Primary Examiner. 

1. AN ELECTRICAL MACHINE COMPRISING A ROTOR ASSEMBLY HAVING PERMANENT MAGNET MEANS AND MOUNTED FOR ROTATION ON AN AXIS EXTENDING CENTRALLY OF SAID PERMANENT MAGNET MEANS, SLEEVE MEANS SURROUNDING SAID PERMANENT MAGNET MEANS AND HAVING AN INTERIOR SURFACE CONFORMING WITH EXTERIOR SURFACE PORTIONS OF SAID PERMANENT MAGNET MEANS, SAID SLEEVE MEANS PROVIDING SEGMENTAL CYLINDRICAL MAGNETIC POLES, AND MEANS OF MAGNETIC MATERIAL AT OP- 